The present invention relates to devices that regulate, condition and control AC power that is provided to an AC load, such as an induction motor. The invention is more specifically directed to an AC power conditioning device that can adjust the wave shape and voltage level of AC power applied to an AC load, as well as power factor and frequency, to compensate for deterioration in quality of the AC line power. The power conditioning device of this invention can be used with motors from fractional horsepower to several horsepower or above, and where the torsional load on the motor may vary depending upon external factors, and in situations where the quality of the line power may drop, i.e., from a nominal 117 VAC (in North America) to below 100 VAC. Also, the device may be employed for controlling the power factor for an AC or inductive load that may vary during use, such as a single-phase AC induction motor, which may be used to drive a compressor in a HVAC application or in a refrigerator.
For any AC motor, the available motor torque can depend on the condition or quality of the AC line power. The output torque is proportional to the square of the input voltage. During many peak demand times, the quality of the AC line power can vary enormously, with changes in line voltage and in line frequency. A drop in line voltage from 117 VAC to 100 VAC (a drop of about 14.5%) results in a reduction in torque of about 27%. Typically, the motor designer is forced to over-design the motor in order to satisfy load requirements over an expected range of input conditions. The motor armature, which is basically an inductive load, may have to face an unfavorable power factor, which means that the actual applied voltage, i.e., the real component of the complex AC voltage, may become unacceptably low. Consequently, it is desirable to adjust the RMS value of the line voltage so that the motor will operate optimally, even under adverse line conditions.
It is also the practice for any given application to use a motor that is rated over a given voltage range of ±10%. This means that the system has to be over-designed to meet full load requirements at low voltage. Otherwise, for a given AC induction motor, if the input voltage is 10% low, i.e., V=90% Vnormal, then output torque T drops to T=81% Tnormal. This means that, according to conventional practice, the motor has to be over-designed by at least 19%. Consequently, at normal or high line conditions, over 20% of the electric energy is wasted or reflected back towards the power station.
One approach to motor control has been a variable frequency drive (VFD) employing a pulse-controlled inverter, intended for control over motor speed. In VFD the incoming AC power is rectified to produce constant DC “rail” levels, and then an inverter converts the DC power to an AC drive wave using pulse-width modulation (PWM). This technique modifies the leading and trailing edges of a square wave produced by the inverter by chopping the power on and off at a very high rate so that the average current wave can approximate a sinusoidal wave. These VFDs overcome some of the difficulties of operating induction motors directly on line voltage, and permit a range of speed control. However, the use of PWM can lead to other problems, including winding insulation failure in the motor armature, and high switching losses. Moreover, the PWM VFD devices do not, per se, boost the voltage.
In many cases, what is needed is simply to boost (or to regulate) the effective RMS voltage. This can permit use of a smaller-rated motor than would be recommended where unmodified line power is applied directly to the motor.
An example of a phase detection power factor controller circuit, which addresses some of these issues, is discussed in Nola U.S. Pat. No. 4,459,528. There, an active power factor converter is discussed, which reduces the effective applied power by use of a thyristor (triac) and turning the thyristor on and off at various phase angles so as to change the shape of the applied power wave and optimize the phase angle or power factor. Another power factor controller is discussed in Bach Published Application No. US 2003/0122433. The device described there is an active power factor controller with power factor correction and also with a soft-start feature for applying a gradually increasing voltage to the AC load at turn on. These are accomplished by switching the applied power to regulate the amount of the AC input power that passes to the load. These can reduce effective applied voltage but do not boost the power (i.e., voltage) applied to the load.
Power factor (phase angle) correction is a problem for both consumers of AC power and commercial providers. The common practice is to place one or more capacitors in parallel to the load (in the case of an inductive device such as a motor armature). The size of the capacitor has to be selected to match the motor impedance, which can change with line and load conditions. This means that a number of capacitors have to be placed in parallel and switched in or out of circuit as conditions change. This technique requires high capacity AC devices, which are bulky and pricey.
What is needed where line power can be too low or too high is a simple, reliable power conditioner that is capable of increasing the AC voltage or decreasing the applied AC voltage, as needed, to optimize the operation of the induction motor or other AC load device. It is also desirable to avoid the high switching rates of power switching components, as discussed above, which can result in damage to the motor and can produce significant RF energy.
Earlier efforts in brownout protection (i.e., to protect the AC induction motor from burn out in low line voltage situations) have typically involved simply cutting off power to the motor to prevent damage. While this saves the motor, it can cause severe problems for the system that the motor is designed to drive. For example, in a commercial refrigeration application, a freezer system may be used for storage of a frozen food product, e.g., frozen meat, ice cream, or another food product. During a so-called brownout, when the operating line voltage drops below a safe threshold (e.g., reduced from 120 volts RMS to below 95 volts RMS) then the compressor motor is simply shut off, and no refrigeration takes place. If the brownout lasts for a period of an hour or more, the meat may begin to spoil, or the ice cream may melt. It would be more desirable to continue to operate the refrigeration system during brownouts, i.e., modifying the AC power wave so that it is sufficient to run the equipment, even if at a partial speed. However, that has not been possible with existing power control circuits.
There are regions where commercial power is not particularly reliable, and where the line power can vary significantly up and down during the day. In such areas, the conventional approaches have involved use of a variable transformer to boost the voltage, and/or an extra heavy-duty motor that is over-designed for reliable power but which is able to withstand significant drops in the AC line voltage without failure. These approaches waste a significant amount of power.